The present invention relates to a semiconductor device and a power converter using it and more particularly, to a semiconductor device of the type having flywheel diodes and a power converter using the same.
In recent years, many kinds of inverters and converters have been used in electric power converters dedicated to energy saving and renewable energy technologies. For realization of Low-carbon society, drastic spread of the power converters is still indispensable. FIG. 19 shows an example of an inverter for controlling a motor 950 in variable speed and realizing energy saving. Electric energy from a DC power supply Vcc is converted into an alternating current at a desired frequency through the use of IGBT's (Insulated Gate Bipolar Transistors) 700, that is, a kind of power semiconductors, in order to change the revolution speed of the motor 950. The motor 950 is a three-phase motor having an input 910 of U-phase, an input 911 of V-phase and an input 912 of W-phase. Input power is fed to the U-phase 910 when a gate circuit 800 for an IGBT 700a having its collector connected to a positive terminal 900 of power supply (hereinafter referred to as an upper arm IGBT) is turned on. On the other hand, by turning off the gate circuit 800, feeding of the input power to the U-phase 910 can be stopped. By repeating this operation, electric power at a desired frequency can be supplied to the motor 950.
A flywheel diode 600a is connected to the IGBT 700a in anti-parallel relation thereto. For example, with the upper arm IGBT 700a turned off, for example, the flywheel diode 600a is so operated as to commutate the current having been flowing through the IGBT 700a to a flywheel diode 600b connected to an IGBT 700b having its emitter connected to a negative terminal 901 of power supply (hereinafter referred to as a lower arm IGBT) in anti-parallel relation thereto, thus making it possible to release energy accumulated in a coil of the motor 950. As the upper arm IGBT 700a is again turned on, the lower arm flywheel diode 600b is rendered non-conductive and electric power is supplied to the motor 950 via the upper arm IGBT 700a. In this manner, the flywheel diodes 600a and 600b are rendered non-conductive and conductive reiteratively in accordance with turn on and off of the IGBT's 700a and 700b and therefore, the conduction loss in each of the flywheel diodes 600a and 600b needs to be decreased for the sake of realizing high efficiency, size reduction and cost reduction of an inverter to thereby promote the widespread use of the inverter. To this end, the forward voltage drop occurring in each of the flywheel diodes 600a and 600b when the current flows through these flywheel diodes must be reduced. In a power semiconductor having a rated voltage of several hundreds volts or more, a pn diode made of silicon to have the ability to increase the conductivity by injecting electric charges is generally used to decrease the forward voltage drop.
On the other hand, when the upper arm IGBT 700a repeats turn on and off, electric charges accumulated during forward biasing in the lower arm flywheel diode 600b are discharged to play the role of a backward recovery current which is superposed on a turn-on current of the upper arm IGBT 700a. The backward recovery current flows through a closed circuit of DC power supply Vcc, parasitic inductance 920, high voltage side terminal 900, upper arm IGBT 700a, lower arm flywheel diode 600b and low voltage side terminal 901 and during the switching, it increases the turn-on loss in the upper arm IGBT 700a and generates the backward recovery loss in the lower arm flywheel diode 600b. If having a large rate of current change (di/dt), the backward recovery current generates an excessive bounce voltage (L×di/dt) cooperatively with the parasitic inductance 920(L) and in case the bounce voltage exceeds the rated voltage of IGBT 700 or flywheel diode 600, the inverter will sometimes become troubled and faulty.
As described above, the pn diode used for each of the flywheel diodes 600a and 600b can on the one hand reduce the forward voltage to succeed in decreasing the conduction loss but on the other hand, increases the backward recovery loss, giving rise to generation of a bounce voltage. In contrast to the pn diode, a Schottky diode is available in which the amount of injected electric charges is small and the backward recovery current is very small. But for silicon diode, the forward voltage is very large and the loss increases in the inverter handling large currents. Recently, a Schottky diode using silicon carbide (SiC) in place of silicon has been noticed. However, this type of Schottky diode is disadvantageous in that its crystalline quality is bad, its fabrication process is difficult and its increase in diameter size is inferior to that using silicon, resulting in high costs leading to prevention of cost reduction of the inverter and converter, and therefore it comes into limited use.
A conventional composite flywheel diode having pn diodes and Schottky diodes both made of silicon is described in Japanese Patent No. 2590284 (corresponding to U.S. Pat. No. 5,101,244) and is now illustrated in FIG. 20 in the accompanying drawings. A semiconductor substrate 1 has a cathode electrode 2 in ohmic contact 11 to an n+ layer 13 and an n− layer 14 overlying the n+ layer 13 forms pn junctions 15 in association with deep p layers. The n− layer 14 cooperates with an electrode 3 to sandwich shallow p layers, forming Schotkky junctions 16. The electrode 3 makes an ohmic contact to the deep p layer and urges the deep p layer to inject electric charges (holes) from it. By coupling the pn junction and the Schottky junction by means of the electrode 3, the amount of injection of electric charges can be increased/decreased in contrast to the case of the presence of either the pn diode alone or the Schottky diode alone, so that the forward voltage can be prevented from becoming drastically large and the backward recovery current can be prevented from extremely increasing and besides the rate of change of current di/dt of backward recovery current can be reduced, thus ensuring that the tradeoff characteristic relation among the reduction in conduction loss, the reduction in switching loss and the suppression of bounce voltage can be improved.
In the diode shown in FIG. 20, however, there arise problems that further injection of electric charges to reduce the forward voltage drop degrades the backward recovery characteristics and conversely, the suppression of electric charge injection aiming at improvements in the backward recovery characteristics increases the forward voltage accompanied by an increase in loss, making it difficult for the conventional structure to further improve the characteristics of the flywheel diode.